1. Field of Invention
The present invention relates to an optical proximity correction method. More particularly, the present invention relates to a contact hole model-based optical proximity correction method.
2. Description of Related Art
As the level of integration in an integrated circuit continues to increase, dimensions of each circuit device must reduce correspondingly. Photolithography is an important step in the fabrication of semiconductors. Photolithography is involved in processes related to the fabrication of metal-oxide-semiconductor (MOST) devices such as the patterning thin films and the marking out areas for implanting dopants. In addition, whether the semiconductor industry is able to produce devices with smaller critical dimensions depends very much on the future development of photolithography. To attain higher level of integration, a few methods capable of increasing mask resolution has been suggested. One such method is optical proximity correction (OPC).
The idea behind optical proximity correction is the eliminate line offset due to proximity effect. Proximity effect refers to the phenomenon that occurs when a beam of light shines through an optical mask and projects onto a wafer. Due to diffraction of light beam on passing a material medium, the light beam will expand out somewhat. Furthermore, some of the light may pass through the photoresist layer into the wafer and then reflect back from the semiconductor substrate leading to interference. Hence, some portion of the photoresist may be double-exposed. The seriousness of such occurrences is intensified when the feature line width of an integrated circuit is small, especially when the wavelength of the light source approaches the width of a line pattern.
At present, contact hole model-based optical proximity correction method relies on test patterns established by the differences in distances and line widths as a database for actual correction.
FIG. 1A is a sketch showing the test patterns used by a conventional contact hole model-based optical proximity correction method.
As shown in FIG. 1A, the square contact hole 102 in test pattern 100 has a line width 104. Distance of separation or pitch from one contact hole 102 to its neighboring contact hole is labeled 106. For example, line width 104 of the square contact hole 102 in test pattern 100 is 0.8 xcexcm and distance of separation between neighboring contact holes is 1.6 xcexcm. Similarly, line width of contact hole 112 in test pattern 110 is 0.84 xcexcm and distance of separation between neighboring contact holes 112 is 1.68 xcexcm. Finally, line width of contact hole 116 in test pattern 114 is 0.88 xcexcm and distance of separation between neighboring contact holes 116 is 1.76 xcexcm. Using a photomask having the test patterns 100, 110 and 114 thereon, a layer of photoresist is exposed and then developed. Thereafter, line widths of various patterns on the developed photoresist layer are measured. Because of proximity effect, contact hole patterns on the wafer are slightly different from the original test patterns on the photomask. Most probably, the corners of the square holes may be rounded and line width may be smaller. After measuring the actual line widths, a line width versus distance of separation graph may be plotted for both the predicted and the actual values.
FIG. 1B is a graph showing the line width versus distance of separation relationship in a conventional contact hole model-based proximity correction method.
As shown in FIG. 1B, when the distance of separation between contact holes is getting smaller, actual line width measured on the photoresist layer is getting longer than the intended line width on the photomask. Conversely, when the distance of separation between contact holes is getting larger, actual line width measured on the photoresist layer is getting closer to the intended line width on the photomask.
Using the established optical proximity correction model shown in FIG. 1B, proper optical mask line widths can supposedly be selected to form the desired line width on the photoresist layer during a contact hole forming process. In practice, the established model can hardly produce the kind of accuracy demanded. This is because proximity effect may vary according to distance of separation, thereby changing the effect on line width. In other words, when the distance of separation is greater than a few times the line width, proximity effect has little influence on line width dimensions. However, as distance of separation is not much different from the line width, proximity effect can affect the ultimate line width dimensions of the contact holes considerably. Hence, a simple distance of separation versus line width relationship can hardly produce the kind of prediction needed for forming contact holes having correct dimensions and position.
Accurate reproduction of contact holes and conductive lines in an integrated circuit is getting more important especially when the semiconductor devices are getting smaller. There is very little tolerance for a misaligned contact hole or a conductive line having a line width too width or too narrow in an integrated circuit. Hence, optical proximity correction is increasingly important in photolithographic processes.
Accordingly, one object of the present invention is to provide a contact hole model that can simulates the actual photolithographic conditions more accurately than a conventional model so that up to standard silicon chip is produced after a photolithographic process. Consequently, productivity is increased and quality is improved.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a contact hole model-based optical proximity correction method. The method utilizes a series of test patterns each having a plurality of contact holes separated by identical distance but a different contact hole line width to serve as a database for establishing a contact hole model. Using the contact hole model, proximity effect that may lead to line width offset can be eliminated. Ultimately, desired dimensions of a device are more accurately reproduced and production errors are minimized.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.